1. Field of the Invention
The present invention relates to a magnetic shielding material for superconducting magnet, which exhibits excellent conductivity at low temperatures of, for example, 77 K or lower, especially cryogenic temperatures of 20 K or lower; and more particularly to a magnetic shielding material which exhibits excellent conductivity even when used in a strong magnetic field of, for example, 1 T or more.
2. Description of the Related Art
A superconducting magnet has been used in various fields, for example, MRIs (magnetic resonance imaging) for diagnosis, NMRs (nuclear magnetic resonance) for analytical use or maglev trains. There have been used, as a superconducting magnet, low-temperature superconducting coils cooled to its boiling point of 4.2 K (Kelvin) using liquid helium, and high-temperature superconducting coils cooled to about 20 K by a refrigerator.
In order to suppress variation in the outside of the magnetic field from exerting an influence on a superconducting magnet, or to suppress the magnetic field generated by a superconducting magnet from exerting an adverse influence on the outside, a magnetic shielding material is usually arranged on the periphery of the superconducting magnet.
Since the magnetic shielding effect can be obtained in a thinner state as electrical resistivity of the magnetic shielding material becomes lower, a material with low resistivity is usually used.
For example, JP H05-144637A discloses that aluminum, copper and alloys thereof can effectively shield variable magnetic field from the outside to decrease AC (alternating-current) loss inside a superconducting coil because of low electrical resistivity thereof.
There has widely been used, as a magnetic shielding material, oxygen-free copper having a purity of 99.99% by mass or more (hereinafter sometimes referred to as “4N” (four nines) and, in the mass percentage notation which indicates a purity, notation is sometimes performed by placing “N” in the rear of the number of “9” which is continuous from the head, for example, purity of 99.9999% by mass or more is sometimes referred to as “6N” (six nines), similarly), which has low electrical resistivity among coppers.
Heretofore, there have been strong demands for miniaturization and weight saving in apparatuses using such a superconducting coil. In order to perform miniaturization and weight saving, it is essential to arrange a magnetic shielding material close to the superconducting coil.
Putting the magnetic shielding material close to the superconducting coil means that the magnetic shielding material is cooled to cryogenic temperatures such as 4.2 K or 20 K as an operating temperature of the superconducting coil, or a boiling point of 77 K of liquid nitrogen or lower, similarly to peripheral materials arranged on the periphery of the superconducting coil. Furthermore, it means the magnetic shielding material is used under the magnetic field from the superconducting coil, namely, it is used in a state where a strong magnetic field of a magnetic flux density of 1 T (Tesla) or more is applied.
Only under the condition of cryogenic temperatures, desired low resistivity can be obtained by using, for example, the above-mentioned copper or aluminum having a purity of 4N class.
However, there is a problem that electrical conductivity is decreased by the magnetoresistance effect in a strong magnetic field of, for example, 1 T or more. It is known that copper has remarkable magnetoresistance effect (namely, electrical resistivity remarkably increases in the magnetic field), and it is also known that aluminum also exhibits large magnetoresistance effect, although not comparable to copper.
A decrease in conductivity caused by the magnetoresistance effect (increase in electrical resistivity) leads to an increase in penetration depth of eddy currents generated by the magnetic field from the outside. Therefore, it is required to increase the thickness of the magnetic shielding material so as to obtain desired magnetic shielding characteristics, resulting in suppression of miniaturization and weight saving of a superconducting apparatus.